A typical vehicle compressed air system, as used for operating air brakes and other devices, for example, includes a compressor that compresses ambient air to fill one or more service reservoirs. Whenever the compressor is loaded and the system is charging (the “charge phase”), compressed air passes through an air dryer and moisture or water is collected by a desiccant in the dryer. Compressed air is also passed through the dryer to a purge reservoir that is connected to the air dryer.
At the end of every charge cycle, the compressor unloads, and the system automatically purges the dryer (the “purge phase”) with a flow of air from a purge reservoir located downstream of the air dryer, through a purge orifice and through the dryer. This purge regenerates the desiccant in the air dryer. A single purge is provided, no matter how long the system had been charging, and no matter how much purging might actually be required for dryer regeneration (based on the amount of water in the dryer). If a long charge time is needed to maintain the brake system, a second dryer is used, adding cost and space requirements on the vehicle.